Taking a slewing drive apparatus for a shovel as an example, background art will be described.
A shovel includes a crawler-type lower propelling body, an upper slewing body rotatably mounted, around a shaft vertical to the ground, on the lower propelling body, and a work attachment mounted on this upper slewing body.
The shovel further includes a slewing drive apparatus rotating the upper slewing body. This slewing drive apparatus includes a hydraulic or electric motor serving as a drive source, and a speed reducer unit reducing speed of rotation force of this motor with a gear speed reduction mechanism and transmitting the reduced rotation force to the upper slewing body, that is, the driven portion.
The motor has a motor shaft, and the speed reducer unit has a speed reducer output shaft connected to the motor shaft. The motor and the speed reducer unit are provided in a row in an axial direction of the slewing drive apparatus (hereinafter referred to as an apparatus-axis direction) such that both center axes (the motor shaft and the speed reducer output shaft) are located along the same line. Additionally, the motor and the speed reducer unit are mounted to an upper frame in such a vertical arrangement that the motor is located at a top.
The speed reducer unit is at least a single-stage planetary gear mechanism including a sun gear, a planetary gear, and a ring gear, for example. Output of this speed reducer unit is transmitted to the upper slewing body through a pinion provided on the speed reducer output shaft, and a slewing gear provided on a lower frame of the lower propelling body.
Lubricating oil for lubricating a gear mechanism such as the planetary gear mechanism is injected in the speed reducer unit. A temperature of this lubricating oil increases by heat generated by the slewing drive apparatus (speed reducer unit). Therefore, the slewing drive apparatus is provided with a cooling means for cooling the lubricating oil.
For example, Patent Document 1 describes a technique of providing a coolant path in a casing of a speed reducer unit as the cooling means, and cooling lubricating oil from an outer periphery by flowing a cooling medium such as water and oil in this coolant path.
However, according to the technique described in Patent Document 1, there are the following problems.
(I) A jacket structure, in which the coolant path is provided so as to have the size in a range of a thickness of a peripheral wall of the casing, is employed. Therefore, the thickness of the peripheral walls of the casing is required to increase, and a diameter of the casing increases.
Accordingly, a maximum diameter of the drive apparatus increases, and a space occupied by the drive apparatus in this diameter direction increases. Therefore, particularly under a situation in which many apparatuses are densely placed in a small space like the slewing drive apparatus for the shovel, the layout of the apparatuses becomes difficult.
(II) In a case of the slewing drive apparatus, a shaft support portion provided on a lower end of the speed reducer unit is bolted on the upper frame from above. Here, according to the technique described in Patent Document 1, the casing of the speed reducer unit increases in diameter by providing the coolant path. Therefore, a tightening tool such as a wrench is difficult to enter due to interference of this casing, and tightening or loosening work of a bolt for mounting the shaft support portion becomes troublesome.
(III) The lubricating oil inside the casing is cooled only from an outer periphery side of the casing. That is, only one surface (inner surface) of the coolant path (casing) contributes to cooling. Therefore, a cooling effect is weak.
(IV) The coolant path is formed so as to have the size in the thickness range of the casing. Therefore, as described in Patent Document 1, it is necessary to employ a particular molding method such as molding using a core for the coolant path. Thus, the manufacturing cost of the casing increases.    Patent Document 1: Japanese Patent Publication No. 4504899